Textured silicon substrate and method

ABSTRACT

A method of texturizing a silicon substrate comprising a) contacting the substrate with an etching solution comprising glycolic acid, b) etching a surface of the substrate thereby forming disruptions in said surface of the substrate, and c) removing the etching solution to yield a texturized substrate, said texturized substrate having a plurality of disruptions in at least one surface with a surface density of disruptions of a minimum of 60 disruptions in a 400 micron square area.

FIELD OF THE INVENTION

The field of invention relates to textured silicon substrates and anetching method to produce the same.

BACKGROUND OF THE INVENTION

Textured silicon substrates are used in the manufacture of photovoltaicand electronic products. Texture etching of polycrystalline silicon,referred herein as mc-silicon or silicon, is a key way of improvingphotovoltaic cell efficiency by trapping more light energy duringphotovoltaic cell operation than untextured silicon. A textured siliconsurface minimizes light reflectance thus maximizing light absorption bythe photovoltaic cell producing more energy. A textured silicon surfaceconsists of minute recesses and projections on the planar level of thesilicon surface, and is typically generated by an etching process.

Conventional texturing etch technology consists of treating mc-siliconwith an etching solution. Etching solutions can be either causticsolution or acidic solutions. Caustic etching solutions typically are anaqueous solution containing an alkali earth metal, optionally with analcohol, and are relatively slow when compared to acidic etchingsolutions. Acidic etching solutions typically contain a combination ofacids, water, and optionally an additive. The concentrations of the acidcomponents can be varied to alter the formation of a pattern of minuterecesses and projections on the surface being etched, but suchvariations can also decrease the uniformity of the light trappingefficiencies of the various portions of the surface.

U.S. Pat. No. 6,340,640 of Nishimoto et al, discloses a method toproduce a solar cell using an etch solution containing HF, HNO₃, and anagent, wherein said agent contains at least one carboxylic acid with amolecular weight higher than acetic acid, or a mixture of phosphoricacid and a carboxylic acid with a molecular weight higher than aceticacid, to provide projections and recesses on a surface of the siliconwafer of a solar cell. The carboxylic acid is at least one of propionicacid, butyric acid, valoric acid, caproic acid, tartaric acid, succinicacid, adipic acid, propane-tricarboxylic acid, an isomer ofpropane-tricarboxylic acid. However, it is known that variations in thequality of the mc-silicon available make it difficult to realizeefficiencies in mass production. The etching solution often needs to becustomized to the quality of the mc-silicon available, requiringadjustments in the concentrations of acids used.

It is desirable to have a texturized silicon wafer with an increasednumber of surface projections and recesses in a continuous pattern ofimproved uniformity, and an increased surface density of suchprojections and recesses, while minimizing light reflectance. Alsoneeded is a method to produce such a silicon substrate withoutdecreasing etch rates acceptable to current manufacturing operations.The present invention meets these needs.

SUMMARY OF THE INVENTION

The present invention comprises a method of texturizing a siliconsubstrate comprising a) contacting the substrate with an etchingsolution comprising glycolic acid, b) etching a surface of the substratethereby forming disruptions in said surface of the substrate, and c)removing the etching solution to yield a texturized substrate.

The present invention further comprises a textured silicon substratecomprising a silicon substrate which has a plurality of disruptions inat least one surface created by contacting said silicon substrate withan acidic etching solution containing glycolic acid.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a photomicrograph (at 3,000 times magnification) of varioussilicon surfaces. FIG. 1A represents a texturized silicon surface of thepresent invention prepared using an etching solution containing glycolicacid. FIG. 1B represents a silicon surface which has not beentexturized. FIG. 1C represents a silicon surface texturized with anetching solution of hydrofluoric acid, nitric acid, and water. FIGS. 1Dthrough 1F represent silicon surfaces which have been texturized usingan etching solution containing hydrofluoric acid, nitric acid, water,and an additional acid. FIG. 1D represents a silicon surface texturizedusing an etching solution containing acetic acid as the additional acid,FIG. 1E represents a silicon surface texturized using an etchingsolution containing maleic acid as the additional acid, and FIG. 1Frepresents a silicon surface texturized using an etching solutioncontaining phosphoric acid as the additional acid.

FIG. 2 is a graph of wavelength vs. light reflectance for siliconsurfaces as described for FIGS. 1C through 1F.

DETAILED DESCRIPTION OF THE INVENTION

Herein all trademarks are designated with capital letters.

All patents cited herein are hereby incorporated by reference.

“Silicon” as used herein means multicrystalline silicon, also denoted asmc-silicon.

“Textured” or “texturized” means the presence of a plurality of minuteinclined planes in a surface due to the presence of disruptions so thatthe surface is uneven.

“Disruptions” means minute projections and recesses in a plane of asurface.

The present invention comprises a method of texturizing a siliconsubstrate comprising a) contacting the substrate with an etchingsolution comprising glycolic acid, b) etching a surface of the substratethereby forming disruptions in said surface of the substrate, and c)removing the etching solution to yield said texturized substrate. Theetching solution used in said method further comprises one or more ofhydrofluoric acid, nitric acid, water, and surfactant. The etchingsolution contains a minimum of 5% by weight of glycolic acid, preferablyfrom about 5% to about 40% by weight glycolic acid.

The present invention further comprises a textured silicon substratecomprising a silicon substrate which has a plurality of disruptions inat least one surface created by contacting said silicon substrate withan etching solution containing glycolic acid. The disruptions aredispersed in a continuous pattern having minimal variations. The surfacedensity of the disruptions is a minimum of 60 disruptions in a 400square micron surface area. Preferably the surface density of thedisruptions is a minimum of 80 disruptions in a 400 square micronsurface area, more preferable a minimum of 100 disruptions in a 400square micron surface area.

Silicon substrates used in the method of the present invention can beproduced by methods known to those skilled in the art, and are commonlymade from crystalline silicon, preferably multicrystalline silicon.Silicon ingots are formed by cooling molten silicon in a form andcutting into substrates having a thickness of several hundredmicrometers. The cutting is typically performed by a wire cutter andresults in a rough surface on the silicon face. The uniformity andsmoothness of the cut surface varies, thus causing variation in thequality of available silicon substrates. The silicon ingot is thentexturized.

The textured silicon substrate of the present invention minimizes lightreflectance. By light reflectance, it is meant that light is scatteredor reflected away from the substrate at the surface, and not adsorbedinto the textured silicon substrate. For photovoltaic cells, a low lightreflectance, preferably at low wavelength, such as, for example 300 to900 nm, is needed for maximum cell efficiency. Increased surfacedisruptions decrease light reflectance. The textured silicon substrateof the present invention contains a maximum light reflectance of about30% at wavelengths of from about 300 to about 900 nm. Preferably themaximum wavelength is about 20%, more preferably about 15%, and morepreferably about 12% at these wavelengths of from about 300 to about 900nm.

The textured silicon substrate of the present invention contains aplurality of disruptions on at least one planar surface. The pattern ofdisruptions affects light reflectance and photovoltaic performance.Variations of the magnitude, height, and spacing of the disruptions willdecrease the light absorbance. Thus it is desirable to minimize thevariations by increasing the uniformity of the pattern. A continuouspattern of disruptions provides maximum light absorbance. The texturedsilicon substrate of this invention contains a plurality of disruptionsdispersed in a continuous pattern having minimal variations.

The textured silicon substrates of the present invention can be ofvarious sizes, thicknesses and shapes. The textured silicon substratescan be a wafer or cut into rectangles or squares. The thickness of thesubstrates can vary for specific uses and is limited to the precision ofcutting instrumentation. The texture silicon substrate of this inventionpreferably is in the form of a wafer or a thin film having a thicknessof at least 2 micrometers, preferably at least 3 micrometers, morepreferably at least 5 micrometers.

The textured silicon substrates of the present invention are useful assemiconductor substrates. These semiconductor substrates can be used,for example, in electronic devices, integrated circuit boards, andphotovoltaic cells. For photovoltaic cells, the increase in lightadsorption of the silicon substrate after texturizing results in higherenergy conversion. Maximum light absorbance, specifically at the lowerwavelengths, such as for example 300 to 900 nm, results in high energyoutputs. The textured silicon substrate of this invention is preferablyin the form of a semiconductor substrate for use in photovoltaic cells.

To produce a textured silicon substrate of this invention, the presentinvention further comprises a method of texturizing a silicon substratecomprising a) contacting the substrate with an etching solutioncomprising glycolic acid, b) etching a surface of the substrate therebyforming disruptions in said surface of the substrate, and c) removingthe etching solution to yield said texture silicon substrate.

The etching solution used in the method of the present invention is madeby physically combining glycolic acid, hydrofluoric acid, nitric acid,with water, and optionally a surfactant. Order of addition of the acidsis not critical, but, for safety, the acids should be added to thewater. Commercially available acids are suitable for use in the etchingsolutions used in the present invention. Typical concentrations ofcommercially available acids in aqueous solution suitable for use in thepresent invention are as follows: hydrofluoric acid at from about 38% to52% by weight, nitric acid at from about 52% to 70% by weight, andglycolic acid at from about 20% to 80% by weight. Etching solutioncompositions useful in the method of the present invention containglycolic acid and vary in the amount of each of the other acids presentand the strength of each acid present.

Specific examples of etching solutions useful in the method of thepresent invention are those containing from about 4% to about 40% byweight of glycolic acid. Preferably glycolic acid is present at fromabout 5% to about 30% by weight, more preferably at from about 5% toabout 20% by weight. The glycolic acid is preferably added to an acidmixture having a ratio of HF:HNO₃:water of 7:3:7; of 7:3:5; of 5:5:7; orof 5:5:9. These ratios are based on use of commercially available HF ata concentration of about 49% by weight in aqueous solution, commerciallyavailable HNO₃ at a concentration of about 70% by weight in aqueoussolution, and commercially available glycolic acid at a concentration ofabout 70% by weight in aqueous solution.

These commercially available concentrations of acids are used ascomponents in the following preferred embodiments. Examples of preferredembodiments include use of the following specific etching solutions(components are listed below as percent by weight of the etchingsolution) in the method of the present invention: 1) the etchingsolution comprises from about 37.4% to about 39.2% hydrofluoric acid,from about 16.0% to about 16.8% nitric acid, from about 37.4% to about39.2% water, and from about 4.8% to about 9.2% by weight glycolic acid;2) the etching solution comprises from about 38.9% to about 42.4%hydrofluoric acid, from about 16.7% to about 18.2 nitric acid, fromabout 27.8% to about 30.3% water, and from about 9.1% to about 16.7% byweight glycolic acid; 3) the etching solution comprises from about 21.0%to about 26.7% hydrofluoric acid, from about 21.0% to about 26.7% nitricacid, from about 29.4% to about 37.4% water, and from about 9.1% toabout 28.6% by weight glycolic acid; and 4) the etching solutioncomprises from about 23.9% to about 25.1% hydrofluoric acid, from about23.9% to about 25.1% nitric acid, from about 42.1% to about 45.1% water,and from about 4.8% to about 9.1% by weight glycolic acid.

Typical surfactants are wetting agents that lower the surface tension ofa liquid. Surfactants are commercially available, for example from E. I.du Pont de Nemours and Company, Wilmington, Del., and Stepan Company,Northfiled Ill. Surfactants comprise between 0% and about 5% by weightof the etching solution used in the method of the present invention.Preferred for use in the present invention are non-ionic surfactants.Examples of specific surfactants useful in the method of the presentinvention include fluorinated ethoxylated non-ionic surfactants. Theseare commercially available as ZONYL FSO, ZONYL FSN-100, and ZONYLFSO-100 available from E. I. du Pont de Nemours and Company, Wilmington,Del.

In the method of this invention said contacting of the silicon substrateand the etching solution can be carried out by immersing the siliconsubstrate into the etching solution or by spraying the substrate withthe etching solution. Preferably, said contacting is carried out byimmersing the substrate into the etching solution, followed by itsremoval from the solution. The immersion and removal can be accomplishedby mechanical means, such as an automated device, wherein said deviceholds a substrate in place, dips the substrate into an etching solutionbath for a set period of time, and removes the substrate from theetching solution. In the method of this invention an etching rate fromabout 0.6 to about 1.3 micrometers per minute is employed. Preferablythe etching rate is kept constant during etching.

The method of this invention further comprises washing the texturedsilicon substrate surface with water, alcohol, or a basic solution, anddrying. The water can be distilled, de-ionized, or untreated, butpreferably the water is free from contaminants. A variety of alcoholsare suitable for use in the present invention. Preferred are simplealcohols containing six or fewer carbons, such as isopropanol. Typicalwashing times are 30 to 60 seconds. Drying can be accomplished by anymeans known to those skilled in the art including, for example, heatdrying, air drying, forced-air drying, and fan drying.

The etching of silicon substrates by solutions containing HF and HNO₃proceeds by an oxidation of the surface silicon atoms accompanied bydissolution of the resulting Si—O species. For etching solutionscontaining a higher concentration of HF, the rate controlling step isthe oxidation-reduction reaction. A high concentration of HF produces asurface which is rough and pitted with sharp peaks. For use of etchingsolutions containing higher levels of HNO₃ dissolution of the formedoxide is the rate controlling step and diffusion of the complexingfluoride species is the important factor. A high concentration of HNO₃produces a smoother surface with more rounded peaks. Thus the relativeconcentration of the acids in the etching solution affect the morphologyof the texturized surface obtained. Additional components in the etchingsolution affect this system. Further the quality of the unetched siliconvaries in that the cutting process and equipment used affect the siliconsurface used as a starting material in the etching process. Thus thereis a need to determine what particular etching solution composition willwork best with the available silicon causing inefficiencies inmanufacturing operations.

While not wishing to be bound by theory it is believed that use ofhydrofluoric acid in the etching solution removes oxide from the surfaceof the silicon substrate rendering the surface hydrophobic and affectingits wettability. Thus there is a need to balance enhanced wettabilitywith increasing surface roughness. The etching solution needs to improvewettability, which is done through use of a carboxylic acid group topromote uniformity, while also still increasing surface area androughness, which can be done through use of a hydroxyl group. Becauseglycolic acid contains the same number of carboxylic and hydroxylradicals, it is believed this makes it more effective in creatingdisruptions to promote light trapping efficiency which are more uniformacross the morphology of the surface.

As can be seen from FIGS. 1 and 2, the use of glycolic acid in theetching solution compared to other acids alters the surface morphology.Large microcracks on the surface of the silicon are removed resulting inmore surface uniformity. FIG. 1A of a surface texturized with an etchingsolution containing glycolic acid shows that the texture of the surfacecontains more and smaller disruptions while maintaining sufficientroughness to produce a low light reflectance. Comparing FIG. 1A withFIG. 1C (no glycolic acid present), and with FIGS. 1D through 1F (otheracids substituted for the glycolic acid) reveals the more uniformmorphology provided when the method of the present invention isemployed. The surface area in FIG. 1A is altered compared to silicontexturized with other acids in place of the glycolic acid in the etchingsolution. FIG. 2 provides a graph of wavelength versus reflectance forthe etched surfaces shown in FIGS. 1A and 1C through 1F. FIG. 2 showsthat use of glycolic acid in the etching solution provides sufficientsurface roughness to maintain low reflectance while FIG. 1A shows thegreater uniformity of the disruptions on the surface. In FIG. 2 see lineA for the solution containing glycolic acid versus lines C (no glycolicpresent) and D through F (other acids substituted for the glycolicacid). Thus in the present invention uniformity and roughness arebalanced. Glycolic acid also provides superior buffering efficiency tothe etching solution thus promoting longer performance time before thesolution needs to be replaced. In addition, use of glycolic acidprovides no residues on the etched surface. Thus use of glycolic acid inetching solutions provides advantages not obtained by use of otheracids.

Further a hydrophobic surface is known to be prone to creation ofwatermarks upon drying. Such watermarks on silicon wafers used insemiconductor processing are undesirable as they are associated withcausing electrical failure. Thus by better balancing of wettability andincreasing surface roughness, use of glycolic acid in etching solutionsprovides texturized silicon surfaces more suitable for use insemiconductor processes. The method and texturized silicon substrates ofthe present invention thus provide advantages in both semiconductorprocessing and photovoltaic processing.

Materials and Test Methods

The following materials and test methods were used the Examples herein.

All wafers used in the following examples are multicrystalline siliconwafers, commercially available from DC Chemical Co., Ltd., Seoul, SouthKorea. Following the texturing of each wafer in the examples andcomparative examples, the wafers were tested for light reflectance,microscopy.

Scanning electron microscopy, referred herein as microscopy, was used todetermine variation in the continuous pattern formed during etching.

Light reflectance spectroscopy was used to determined light trappingefficiency. UV-Vis-NIR spectra were collected on a UV-3101 PC,commercially available from Shimadzu, Kyoto, Japan, and used BaSO₄ as areference.

For preparation of etching solutions, hydrofluoric acid (HF, 49% byweight aqueous solution), nitric acid (HNO₃, 70% by weight aqueoussolution), and either glycolic acid (70% by weight aqueous solution) orphosphoric acid or maleic acid or acetic acid, were mixed with water atconcentrations stated and cooled to 25° C. prior to use.

EXAMPLES Examples 1 to 4

Etching solutions were prepared by mixing HF (49% in aqueous solution),HNO₃ (70% in aqueous solution), and water, in a ratio of 7:3:7 byweight, and adding 5%, 10%, 15%, and 20% by weight glycolic acid,denoted as Examples 1-4 respectively. Compositions of the etch solutionsare listed in Table 1 by weight percent of each component in the finalsolutions. Silicon wafers were etched by immersion into each of theetching solutions for 180 seconds at 25° C. The wafers were removed fromthe etch solution, rinsed in deionized water, and were dried usingforced-air drying. The wafers were tested for light reflectance. Lightreflectance results are listed in Table 2. Example 2 was also analyzedusing microscopy. FIG. 1 shows the micrograph of Example 2 denoted asarea A. FIG. 2 shows the resulting graph of wavelength vs. lightreflectance denoted as line A.

Comparative Example A

A silicon wafer, as used in Examples 1-4, was not contacted with anyetching solution (untexturized) and was subjected to light reflectanceand microscopy. FIG. 1 shows the microscopy denoted as area B. Lightreflectance data is shown in Table 2.

Comparative Example B

An etching solution was prepared as in Examples 1 to 4 but no glycolicacid was added. The composition of the etch solution is listed in Table1 by weight percent of each component in the final solution. A siliconwafer was etched by immersion into the etching solution for 180 secondsat 25° C. The wafer was removed from the etch solution, rinsed indeionized water, and dried using forced-air drying. The wafer was testedfor light reflectance and microscopy. Light reflectance results arelisted in Table 2. FIG. 1 shows the microscopy denoted as area C.

TABLE 1 Glycolic HF HNO₃ Water Acid Example % by weight 1 39.2 16.8 39.24.8 2 37.4 16.0 37.4 9.1 3 35.8 15.3 35.8 13.0 4 34.3 14.7 34.3 16.7 A0.0 0.0 0.0 0.0 B 41.2 17.6 41.2 0.0

TABLE 2 Percent Light Reflectance Wavelength (nm) Example 900 800 700600 500 400 300 1 6.204 5.469 4.977 4.697 4.526 3.816 3.223 2 9.6077.884 6.279 4.684 3.638 4.059 3.038 3 9.831 7.468 5.83 4.146 3.43 6.4196.699 4 10.67 9.375 8.136 6.473 4.55 3.227 2.98 Comp A 24.51 26.58 28.0729.72 32.68 39.96 46.31 Comp B 6.895 6.57 6.822 6.616 6.07 5.917 3.543

Results for Examples 1 though 4 showed a reduction of light reflectanceacross 300 to 900 nm compared to the un-textured silicon wafer ofComparative Example A. Examples 1 though 4 also showed an overallimprovement of light reflectance, specifically across wavelengths 500 nmto 700 nm where the light reflectance percentages were reduced by asmuch as 3% when compared to Comparative Example B, an etch solution ofsimilar acid strength without glycolic acid.

Examples 5 to 8

Etching solutions were prepared by mixing HF (49% in aqueous solution),HNO₃ (70% in aqueous solution), and water, in a ratio of 7:3:5 byweight, and adding 5%, 10%, 15%, and 20% by weight glycolic acid,denoted as Examples 5 to 8 respectively. Compositions of the etchsolutions are listed in Table 3 by weight percent of each component inthe final solutions. Silicon wafers were etched by immersion into eachof the etching solutions for 180 seconds at 25° C. The wafers wereremoved from the etch solution, rinsed in deionized water, and weredried using forced-air drying. The wafers were tested for lightreflectance. Light reflectance results are listed in Table 4.

Comparative Example C

An etching solution was prepared as in Examples 5 to 8 but no glycolicacid was added. The composition of the etch solution is listed in Table3 by weight percent of each component in the final solution. A siliconwafer was etched by immersion into the etching solution for 180 secondsat 25° C. The wafer was removed from the etch solution, rinsed indeionized water, and dried using forced-air drying. The wafer was testedfor light reflectance. Light reflectance results are listed in Table 4.

TABLE 3 Glycolic HF HNO₃ Water Acid Example % by weight 5 44.4 19.0 31.74.8 6 42.4 18.2 30.3 9.1 7 40.6 17.4 29.0 13.0 8 38.9 16.7 27.8 16.7Comp C 46.7 20.0 33.3 0.0

TABLE 4 Percent Light Reflectance Wavelength (nm) Example 900 800 700600 500 400 300 5 10.48 9.981 10.4 10.96 11.53 12.47 9.425 6 9.676 9.76410.3 10.47 9.947 9.79 5.673 7 8.597 8.263 8.199 8.481 8.597 8.192 4.2018 6.918 7.011 7.123 7.533 7.773 7.567 3.195 Comp C 11.74 10.54 10.029.848 10.33 12.13 8.238

Results for Examples 5 though 8 showed an overall improvement of lightreflectance. Examples 7 and 8, which were etched with etch solutionsthat contained glycolic acid levels of 13% and 17% by weight ofsolution, showed greater reduction in light reflectance, specificallyacross wavelengths 300 nm to 900 nm where the light reflectancepercentages were reduced by as much as 5% when compared to ComparativeExample C, a wafer etched with an etch solution of similar acid strengthwithout glycolic acid,

Examples 9 to 15

Etching solutions were prepared by mixing HF (49% in aqueous solution),HNO₃ (70% in aqueous solution), and water, in a ratio of 5:5:7 byweight, and adding 5%, 10%, 15%, 20%, 25%, 30% and 40% by weightglycolic acid, denoted as Examples 9 to 15 respectively. Compositions ofthe etch solutions are listed in Table 5 by weight percent of eachcomponent in the final solutions. Silicon wafers were etched byimmersion into each of the etching solutions for 180 seconds at 25° C.The wafers were removed from the etch solution, rinsed in deionizedwater, and were dried using forced-air drying. The wafers were testedfor light reflectance. Light reflectance results are listed in Table 6.

Comparative Example D

An etching solution was prepared as in Examples 9 to 15 but no glycolicacid was added. The composition of the etch solution is listed in Table5 by weight percent of each component in the final solution. A siliconwafer was etched by immersion into the etching solution for 180 secondsat 25° C. The wafer was removed from the etch solution, rinsed indeionized water, and dried using forced-air drying. The wafer was testedfor light reflectance. Light reflectance results are listed in Table 6.

TABLE 5 Glycolic HF HNO₃ Water Acid Example % by weight 9 28.0 28.0 39.24.8 10 26.7 26.7 37.4 9.1 11 25.6 25.6 35.8 13.0 12 24.5 24.5 34.3 16.713 23.5 23.5 32.9 20.0 14 22.6 22.6 31.7 23.1 15 21.0 21.0 29.4 28.6Comp D 29.4 29.4 41.2 0.0

TABLE 6 Percent Light Reflectance Wavelength (nm) Example 900 800 700600 500 400 300 9 11.050 10.340 10.110 10.480 11.560 13.78 8.072 107.863 7.404 7.718 7.646 6.944 6.084 2.559 11 6.476 6.027 6.059 6.0425.814 5.064 2.417 12 7.458 7.034 6.535 6.435 6.947 7.73 2.484 13 4.8784.964 5.295 5.064 4.105 3.398 2.336 14 5.879 5.217 4.820 4.652 4.6013.972 2.591 15 9.102 7.356 6.024 5.113 4.903 5.164 2.78 Comp D 11.9309.517 9.685 9.576 9.717 10.56 6.557

Results for Examples 9 though 15 showed an overall improvement of lightreflectance. Examples 10 through 15, etched with an etch solution whichcontained glycolic acid at levels between 5% and 29% by weight ofsolution, showed greater reduction in light reflectance, specificallyacross wavelengths 300 nm to 900 nm where the light reflectancepercentages were reduced by as much as 6% when compared to ComparativeExample D, a wafer etched with an etch solution of similar acid strengthwithout glycolic acid,

Examples 16 to 19

Etching solutions were prepared by mixing HF (49% in aqueous solution),HNO₃ (70% in aqueous solution), and water, in a ratio of 5:5:9 byweight, and adding 5%, 10%, 15%, and 20% by weight glycolic acid,denoted as Examples 16 to 19 respectively. Compositions of the etchsolutions are listed in Table 7 by weight percent of each component inthe final solutions. Silicon wafers were etched by immersion into eachof the etching solutions for 180 seconds at 25° C. The wafers wereremoved from the etch solution, rinsed in deionized water, and weredried using forced-air drying. The wafers were tested for lightreflectance. Light reflectance results are listed in Table 8.

Comparative Example E

An etching solution was prepared as in Examples 16 to 19 but no glycolicacid was added. The composition of the etch solution is listed in Table7 by weight percent of each component in the final solution. A siliconwafer was etched by immersion into the etching solution for 180 secondsat 25° C. The wafer was removed from the etch solution, rinsed indeionized water, and dried using forced-air drying. The wafer was testedfor light reflectance and microscopy. Light reflectance results arelisted in Table 8.

TABLE 7 Glycolic HF HNO3 Water Acid Example % by weight 16 25.1 25.145.1 4.8 17 23.9 23.9 43.1 9.1 18 22.9 22.9 41.2 13.0 19 21.9 21.9 39.516.7 Comp E 26.3 26.3 47.4 0.0

TABLE 8 Percent Light Reflectance Wavelength (nm) Example 900 800 700600 500 400 300 16 5.782 5.309 4.913 4.745 4.445 3.97 2.571 17 6.2196.020 5.896 5.862 5.910 4.991 2.721 18 6.435 5.528 5.016 4.552 4.3213.54 3.062 19 7.967 6.389 5.319 4.507 4.694 5.191 2.988 Comp E 6.3206.445 6.079 6.102 6.113 5.882 2.885

Results for Examples 16 though 19 showed an overall improvement of lightreflectance compared to Comparative Example E containing no glycolicacid. Examples 16 through 19, wafers etched with an etch solution whichcontained glycolic acid levels between 6 and 18% by weight of solution,showed greater reduction in light reflectance, specifically acrosswavelengths 300 nm to 900 nm where the light reflectance percentageswere reduced by as much as 2% when compared to Comparative Example E, awafer etched with an etch solution of similar acid strength withoutglycolic acid.

Comparative Example F

An etching solution was prepared by mixing HF (49% in aqueous solution),HNO₃ (70% in aqueous solution), water, and acetic acid (99.7% in aqueoussolution) in a ratio of 7:3:7:1 by weight. The composition of the etchsolution is listed in Table 9 by weight percent of each component in thefinal solution. A silicon wafer was etched by immersion into the etchingsolution for 180 seconds at 25° C. The wafer was removed from the etchsolution, rinsed in deionized water, and were dried using forced-airdrying. The wafer was tested for light reflectance and microscopy. Lightreflectance results are listed in Table 10. FIG. 1 shows the micrographdenoted as area D. FIG. 1 shows the resulting graph of wavelength vs.light reflectance denoted as line D.

Comparative Example G

An etching solution was prepared by mixing HF (49% in aqueous solution),HNO₃ (70% in aqueous solution), water, and maleic acid (99% in aqueoussolution) in a ratio of 7:3:7:1 by weight. The composition of the etchsolution is listed in Table 9 by weight percent of each component in thefinal solution. A silicon wafer was etched by immersion into the etchingsolution for 180 seconds at 25° C. The wafer was removed from the etchsolution, rinsed in deionized water, and were dried using forced-airdrying. The wafer was tested for light reflectance and microscopy. Lightreflectance results are listed in Table 10. FIG. 1 shows the micrographdenoted as area E. FIG. 1 shows the resulting graph of wavelength vs.light reflectance denoted as line E.

Comparative Example H

An etching solution was prepared by mixing HF (49% in aqueous solution),HNO₃ (70% in aqueous solution), water, and phosphoric acid (85% inaqueous solution) in a ratio of 7:3:7:1 by weight. The composition ofthe etch solution is listed in Table 9 by weight percent of eachcomponent in the final solution. A silicon wafer was etched by immersioninto the etching solution for 180 seconds at 25° C. The wafer wasremoved from the etch solution, rinsed in deionized water, and weredried using forced-air drying. The wafer was tested for lightreflectance and microscopy. Light reflectance results are listed inTable 10. FIG. 1 shows the micrograph denoted as area F. FIG. 1 showsthe resulting graph of wavelength vs. light reflectance denoted as lineF.

TABLE 9 HF HNO3 Water Acid Example % by weight Acid Used 2 37.4 16.037.4 9.1 Glycolic Acid Comp A 0 0 0 0 none Comp B 41.2 17.6 41.2 0.0none Comp F 23.9 23.9 43.1 9.1 Phosphoric Acid Comp G 23.9 23.9 43.1 9.1Acetic Acid Comp H 23.9 23.9 43.1 9.1 Maleic Acid

TABLE 10 Percent Light Reflectance Wavelength (nm) Example 900 800 700600 500 400 300 2 9.607 7.884 6.279 4.684 3.638 4.059 3.038 Comp A 24.5126.58 28.07 29.72 32.68 39.96 46.31 Comp B 6.895 6.57 6.822 6.616 6.075.917 3.543 Comp F 6.799 6.989 7.126 6.792 5.928 5.417 3.304 Comp H8.763 7.846 7.236 6.488 6.053 6.927 4.768 Comp G 6.946 6.415 6.223 6.2486.696 7.449 3.795

Example 2, Comparative Examples A, B, F, H, and G were tested andcompared in morphology and light reflectance. Morphology micrographs areillustrated in FIG. 1. Light reflectance data is listed in Table 10 andgraphed vs. wavelength in FIG. 2. Example 2 showed an overall reductionof light reflectance, specifically across 300 to 700 nm compared toComparative Example A (a non-textured silicon wafer), ComparativeExample B (a silicon wafer etched with an etch solution of similar acidstrength without glycolic acid), and Comparative Examples F, H, and Gwhere different acids were used in place of the glycolic acid.

Results for microscopy and light reflectance showed that Example 2demonstrated a qualitative change in surface texture, with a highdensity of very small features of about 100 disruptions in a 400 squaremicron area and low light reflectance. These are seen in the micrographsin FIG. 1 and the light reflectance data shown in Table 10 and FIG. 2.The disruptions of Example 2 had features significantly smaller than thedimensions from the Comparative Examples B through D, which had typicaldimensions of less than 1 micrometer by less than10 micrometer, with asurface density of approximately 30 disruptions in a 400 square micronarea.

Consideration of the results from both tests demonstrated that thetextured silicon substrates of this invention and the method oftexturing of this invention provided a high surface density ofdisruptions on the surface and a low light reflectance. This low lightreflectance and high surface density equates to an increase in adsorbedlight into the silicon substrate. For photovoltaic cell applications,increased light adsorption results in an increase of energy produced.

What is claimed is:
 1. A method of texturizing a silicon substratecomprising a) contacting the substrate with an etching solutioncomprising from about 4% to about 40% by weight glycolic acid, b)etching a surface of the substrate thereby forming disruptions in saidsurface of the substrate, and c) removing the etching solution to yielda texturized substrate.
 2. The method of claim 1 wherein the etchingsolution further comprises one or more of hydrofluoric acid, nitricacid, water, and surfactant.
 3. The method of claim 1 wherein theetching solution contains a minimum of 5% by weight of glycolic acid. 4.The method of claim 1 wherein the etching solution contains from about5% to about 28.6% by weight glycolic acid.
 5. The method of claim 1further comprising washing the texturized substrate surface with wateror a basic solution, and drying.
 6. The method of claim 1 wherein thecontacting is by immersing the substrate into the etching solution or byspraying the etching solution onto the substrate.
 7. The method of claim1 wherein the etching rate is from about 0.6 to about 1.3 micrometersper minute.
 8. The method of claim 1 wherein the etching rate is keptconstant during etching.
 9. The method of claim 1 wherein the etchingsolution comprises on a weight percent basis from about 37.4% to about39.2% hydrofluoric acid (49% concentration in aqueous solution), fromabout 16.0% to about 16.8% nitric acid (70% concentration in aqueoussolution), from about 37.4% to about 39.2% water, and from about 4.8% toabout 9.1% glycolic acid (70% concentration in aqueous solution). 10.The method of claim 1 wherein the etching solution comprises on a weightpercent basis from about 38.9% to about 42.4% hydrofluoric acid (49%concentration in aqueous solution), from about 16.7% to about 18.2%nitric acid (70% concentration in aqueous solution), from about 27.8% toabout 30.3% water, and from about 9.1% to about 16.7% glycolic acid (70%concentration in aqueous solution).
 11. The method of claim 1 whereinthe etching solution comprises on a weight percent basis from about21.0% to about 26.7% hydrofluoric acid (49% concentration in aqueoussolution), from about 21.0% to about 26.7% nitric acid (70%concentration in aqueous solution), from about 29.4% to about 37.4%water, and from about 9.1% to about 28.6% glycolic acid (70%concentration in aqueous solution).
 12. The method of claim 1 whereinthe etching solution comprises on a weight percent basis from about23.9% to about 25.1% hydrofluoric acid (49% concentration in aqueoussolution), from about 23.9% to about 25.1% nitric acid (70%concentration in aqueous solution), from about 43.1% to about 45.1%water, and from about 4.8% to about 9.1% glycolic acid (70%concentration in aqueous solution).
 13. A method of texturizing asilicon substrate comprising a) contacting the substrate with an etchingsolution comprising about 4% to about 40% by weight glycolic acid, andone or more of hydrofluoric acid, nitric acid, water, and surfactant, b)etching a surface of the substrate thereby forming disruptions in saidsurface of the substrate, and c) removing the etching solution to yielda texturized substrate.
 14. The method of claim 13, wherein the etchingsolution contains a minimum of about 5% by weight of glycolic acid. 15.The method of claim 13, further comprising washing the texturizedsubstrate surface with water or a basic solution, and drying.
 16. Themethod of claim 13, wherein the contacting is by immersing the substrateinto the etching solution or by spraying the etching solution onto thesubstrate.
 17. The method of claim 13, wherein the etching rate is fromabout 0.6 to about 1.3 micrometers per minute.
 18. The method of claim13, wherein the etching rate is kept constant during etching.
 19. Themethod of claim 2 wherein the etching solution contains from about 5% toabout 28.6% by weight glycolic acid.